How to Calculate Magnetic Field in a Toroid Using Ampere's Law?

[sfdevil]

Homework Statement

Use Ampere’s law to calculate the magnetic field inside of a toroid with 850 turns. The inner radius of the rectangular coils is 3.00 cm, the outer radius is 7.00 cm.

Homework Equations

ampere's law: path integral(B*ds) over a closed path is equal to the enclosed current times mu_0

The Attempt at a Solution

by ampere's law for toroids, I get B= mu_0*N*I/(2*pi*R)
Im confused as to what the inner and outer radius have to do with this problem,
hence the cry for help :(
anyone ?

 

[Kurdt]

The two radii are there so you can determine the axis of toroidal coils. This is chosen as it simplifies Amperes circuital law to the equation you have stated.

 

[sfdevil]

so basically these numbers don't really matter, right ?
since I am finding the magnetic field inside the toroid, its just going to be r as an arbitrary length from the center to some point inside (between 3 and 7 cms) ...correct ?

if so, then the final solution is mu_0*850*I/(2*pi*R), right ?

 

[Kurdt]

If you have a look at the following page it may become clearer.

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/toroid.html

 

[sfdevil]

so from that explanation, I got that since inner radius is 3 and outer is 7, then radius of the rectangular cross-section is 4/2=2
so r=3+2=5

thats about as much as I could get from that page, please let me know If I am still missing something...

 

[Kurdt]

That should be ok.